1. Field of the Invention
This invention relates to a diffraction camera type two-dimensional imaging device for x-ray/gamma ray/neutron radiating object.
2. Description of the Prior Art
A major portion of the field of nuclear medicine involves the use of penetrating radiation (x-rays, gamma rays, or neutrons) to diagnose the presence of tumors in the body or to study organ function. Most often this involves the introduction into the body of tracer quantities of radioisotopes with subsequent imaging of the gamma or x-radiation emitted in their decay, e.g. introduction of radio-iodine has unique capabilities in studies of the function of the thyroid or the indication of the presence of nodules or other abnormalities. Other imaging techniques involve the irradiation of organis with penetrating radiation with subsequent imaging of non-radioactive elements present in the body, e.g. Americium-241 gamma rays have been used to stimulate production of characteristic iodine x-rays in the thyroid which are subsequently imaged for diagnostic purposes.
Certain constraints exist on such techniques which have substantially limited their capabilities:
(a) There are physiological limits on the quantities of radio-pharmaceuticals which may be used, hence the gamma or x-ray luminosity of the organ or function to be imaged is weak.
(b) Conventional optical systems (lenses) cannot be used since x-rays or gamma rays cannot be refracted practically. Hence more primitive optical systems have been used such as pin-hole cameras, mechanical collimators with scanning techniques, etc. These are characterized by low efficiency for imaging and limited resolution capability, or both. Some efforts have been made to develop imaging systems based on Fresnel zone-plate lenses or glancing incidence x-ray optics. These are invariably characterized by very low efficiency or a very small field-of-view.
Diffraction cameras and monochromators have also been used; however, most of the interest in curved crystal, diffraction optical systems has been in the fields of crystal structure studies, x-ray/gamma ray energy spectrometers, x-ray microscopy of surfaces, studies of molecular structure via observation of diffraction patterns, etc. Hence, most interest has resided in the production of a very fine point focus of monochromatic x-radiation. There has been little, if any, interest in using such optics to produce large images.
There is also known a (Berreman, DuMond, and Marmier, Rev. Sci. Inst. 25 1219 (1954) and D. W. Berreman, Rev. Sci. Inst. 26, No. 11, 1048 (1955) system which uses a doubly-bent single crystal of quartz to monochromatize and focus an x-ray beam to an extremely small point for x-ray diffraction studies. The 0.052 planes of quartz were used, the crystal was 3.8 cm.times.7.6 cm.times.0.025 cm thick, ingeniously bent to two different radii of curvature to produce a point image less than 0.5 mm in diameter at one meter from the crystal. The authors observed 2.78% reflection coefficient for Cu K.sub..alpha. radiation at .theta..sub..beta. =71.degree.42'. Using the 0.023 planes of quartz, Berreman calculated the reflection coefficient should be 1.6%. The devices they describe are based on the following physical concepts:
If a single flat crystal with atomic planes parallel to the large surface is bent to conform to the surface of a sphere of radius 2R and if it is ground to fit snugly on a sphere of radius R, then a luminous x-ray object point on the (Rowland) sphere at point P.sub.1 will be exactly focused by Bragg diffraction to a point image at P.sub.2, also on the sphere. All of the crystal will contribute to the image with a reflection coefficient .rho.. This is frequently called the Johansson geometry.